Horton

Re: PoC/WIP: Extended statistics on expressions

Dean Rasheed <dean.a.rasheed@gmail.com>

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From: Dean Rasheed <dean.a.rasheed@gmail.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2020-12-11T12:58:36Z
Lists: pgsql-hackers 
On Tue, 8 Dec 2020 at 12:44, Tomas Vondra <tomas.vondra@enterprisedb.com> wrote:
>
> Possibly. But I don't think it's worth the extra complexity. I don't
> expect people to have a lot of overlapping stats, so the amount of
> wasted space and CPU time is expected to be fairly limited.
>
> So I don't think it's worth spending too much time on this now. Let's
> just do what you proposed, and revisit this later if needed.
>

Yes, I think that's a reasonable approach to take. As long as the
documentation makes it clear that building MCV stats also causes
standard expression stats to be built on any expressions included in
the list, then the user will know and can avoid duplication most of
the time. I don't think there's any need for code to try to prevent
that -- just as we don't bother with code to prevent a user building
multiple indexes on the same column.

The only case where duplication won't be avoidable is where there are
multiple MCV stats sharing the same expression, but that's probably
quite unlikely in practice, and it seems acceptable to leave improving
that as a possible future optimisation.

Regards,
Dean

Commits

  1. Disallow extended statistics on system columns

  2. Identify simple column references in extended statistics

  3. Don't print extra parens around expressions in extended stats

  4. Change position of field "transformed" in struct CreateStatsStmt.

  5. Add transformed flag to nodes/*funcs.c for CREATE STATISTICS

  6. Stabilize stats_ext test with other collations

  7. Extended statistics on expressions

  8. Reduce duration of stats_ext regression tests

  9. Allow composite types in catalog bootstrap

  10. Convert Typ from array to list in bootstrap

  11. Disallow CREATE STATISTICS on system catalogs